Method of producing magnesium base alloys



base alloys containing thorium, e. g.

rates Edward Frederick Emley and William Unsworth, Manchester, England,assignors to Magnesium Elektron Limited, Manchester, England No Drawing.Application February 13, 1953, Serial No. 336,868

Claims priority, application Great Britain October 7, 1952 8 Claims.(Cl. 75-468) This invention relates to the production of magnesiumalloyscontaining thorium, zinc, and zirconium, in accordance with U. S. PatentNo. 2,604,396. These alloys may contain small quantities of rare earthmetals, in fact the commercial thorium or thorium compounds used forintroducing the required thorium content into the magnesium arefrequently contaminated with rare earth metals which are accordinglyintroduced into the magnesium at the same time. Moreover withreprocessing of scrap the rare earth metal content of such alloys mayincrease considerably. However, we have found that it is desirable incertain cases for the alloys to be free from rare earth metals. The mainobject of the present invention therefore is to etfect the removal ofrare earth metals from magnesium base alloysof the kind in questionwithout appreciable loss of the thorium content. It is already knownthat fluxes used for the treatment of magnesium base alloys andcontaining magnesium chloride will cause a reduction of the rare earthmetal content by reaction with the magnesium chloride. However, it hasheretofore been considered that the thorium content is also reduced bythe use of such fluxes due to the reaction of thorium with the magnesiumchloride. Thus in the paper entitled Properties of magnesium-thorium andmagnesium-thorium-cerium alloys, Journal of Metals, March 1952, page287:

It was observed that the behaviour of thorium in molten magnesium withrespect to reaction with the protective fluxes is similar to that ofrare earth metals. In order to obtain the highest possible alloyingefficiency it is necessary to use non-MgClz fluxes.

This point of view appeared also to be borne out by a study of the heatsof formation of thorium chloride and magnesium chloride. Thus it appearsthat the heat of formation of thon'urn chloride is 335,000 cals./moi.compared with 153,300 cals./mol. for magnesium chloride (see MetalsReference Book, Smithells, 1949, pages 434- The heat of formation ofthorium chloride is therefore over 7,000 cals. higher than that ofmagnesium chloride per gram equivalent. Accordingly there would appearto be no possibility of removing the rare earth metals from magnesiumalloys by magnesium chloride without the simultaneous removal of thethorium.

We have however now carried out a very careful study or" the thoriumlosses from magnesium base alloys and have made the surprising discoverythat the losses of thorium which occur on remelting and processing themolten alloys with fluxes containing MgClz are not due to the reactionwith magnesium chloride but arise from oxidation, and that the thoriumcontent is not appreciably reduced by treatment even with magnesiumchloride alone provided that suiiicient care is taken to avoidoxidation. We have further applied this discovery to the possibility ofremoving rare earth metals from magnesium-thorium alloys with extremelysatisfactory results. According to the present invention, therefore,magnesium base alloys containing thorium and one or more rare atent2,745,741 Patented May 15, 1956 ice in the specification of Britishpatent application No. 5,366/52, for instance:

Percent BaClz 40 MgClz 30 KCl 10 CaFz 20 For example an alloy containing1.4% thorium and 0.2-0.5% rare earth metals may be treated withmagnesium chloride thereby reducing the rare earth metal content to lessthan 0.05% Without loss of more than about 0.1% of thorium. In practicethe rare earth metal content can be practically completely eliminatedWithout appreciable loss of thorium.

While the invention may be carried out by stirring magnesium chloride,or a mixture of salts containing magnesium chloride, into the moltenalloy, we have found that it is preferable to pour the molten alloy intoa melt of magnesium chloride or mixture of salts containing magnesiumchloride. This salt melt may conveniently be about 28% of the weight ofthe alloy to be treated and moreover the treatment may be repeated two,three or more times. That is to say, the molten alloy may be poured intothe molten salt in quantity about 5% of the weight of the alloy andstirred therewith; the molten alloy is then poured into a fresh saltmelt of similar quantity, stirred therewith, and then again separatedfrom the salt.

The salt mixture may conveniently consist of magnesium chloride togetherwith potassium chloride. Other salts may also be used such as thechlorides of. other alkali metals and alkaline earth metals, andmagnesium fluoride may be added to promote a clean separation of metalfrom flux. Other fluorides however and magnesium oxide should beomitted. in particular BaClz may be added in amounts not less than 15%by weight of the total mixture (e. g. 15 to 50%) in order to avoidintroducing persistent chloride contamination in the alloy where thelatter contains zirconium in addition to thorium.

We have, therefore, found that it is desirable to carry out thistreatment at a comparatively low temperature, e. g. 650 C.750 C., theremoval of rare earth metals being more effective at such temperaturesthan at higher temperatures. The preferred temperature range is 680C.-720 C.

As an alternative to the use of MgClz we have found that some removal ofrare earth metals from Mg-Th alloys may be effected by bubbling chlorinethrough the molten alloy, but this process is much less effective.

We claim:

1. A method of reducing the content of rare earth metals in magnesiumbase alloys containing both thorium and rare earth metals whichcomprises effecting intimate contact of magnesium chloride with thealloy throughut the body of the alloy, while said alloy is in the moltenstate, and maintaining over the alloy a substance capable of minimisingthe tendency of the alloy to oxidation, such substance being selectedfrom the groupconsisting of metal chloride fluxes, inert atmospheres,and combinations thereof.

2. A method as claimed in claim 1 wherein the alloy is maintained at atemperature of 650 C. to 750 C. whilst such contact is efiected.

3. A method as claimed in claim 1 wherein the alloy is maintained at atemperature of 680 C. to 720 C. whilst such contact is effected.

4. A method as claimed in claim 1 wherein the magnesium chloride isapplied to the alloy in admixture with BaClz in quantity at least 15 percent by weight of the total mixture.

5. A method as claimed in claim 1 wherein the magnesium chloride isapplied to the alloy in admixture with at least two other chlorides ofthe alkali and alkaline earth metals including CaClz in quantity'atleast 15 per cent by Weight of the total mixture.

6. A method as claimed in claim 1 wherein the magnesium chloride isapplied to the alloy in admixture with magnesium fluoride and at leasttwo other chlorides of the alkali and alkaline earth metals includingBaClz in quantity at least 15 per cent by weight of the total mixture.

7. A method as claimed in claim 1 wherein the molten alloy is pouredinto a crucible containing molten mag nesium chloride, and a furtherquantity of magnesium chloride is then stirred into the alloy.

8. A method of reducing the content of rare earth metals in magnesiumbase alloys containing thorium, zirconium and rare earth metals whichcomprises pouring the molten alloy into a crucible containing a saltmixture in molten condition and subsequently stirring into the alloy afurther quantity of such salt mixture, said salt mixture containing atleast 25 per cent of magnesium chloride and at least 15 per cent ofbarium chloride, said mixture being free from magnesium oxide and freefrom fluorides other than magnesium fluoride, a characteristic of suchmixture being that it floats upon the magnesium base alloy and thereforeminimizes the tendency of the alloy to oxidation.

References Cited in the file of this patent UNITED STATES PATENTS2,371,531 McDonald Mar. 13, 1945 2,497,530 Emley Feb. '14, 19502,569,477 Leontis Oct. 2, 1951 2,604,394 Emley July 22, 1952 OTHERREFERENCES Materials and Methods, February 1946, pages 418-424. Journalof Metals, March 1952, pages 287-294.

1. A METHOD OF REDUCING THE CONTENT OF RARE EARTH METALS IN MAGNESIUMBASE ALLOYS CONTAINING BOTH THORIUM AND RARE EARTH METALS WHICHCOMPRISES EFFECTING INTIMATE CONTACT OF MAGNESIUM CHLORIDE WITH THEALLOY THROUGH OUT THE BODY OF THE ALLOY, WHILE SAID ALLOY IS IN THEMOLTEN STATE, AND MAINTAINING OVER THE ALLOY A SUBSTANCE CAPABLE OFMINIMISING THE TENDENCY OF THE ALLOY TO OXIDATION, SUCH SUBSTANCE BEINGSELECTED FROM THE GROUP CONSISTING OF METAL CHLORIDE FLUXES, INERTATMOSPHERES, AND COMBINATIONS THEREOF.